This invention relates to wireless network receivers, and in particular, to an apparatus and method for setting the gain in a radio receiver to take into account any filtering in the radio receiver signal path. The radio may be the radio of a wireless local area network (WLAN) node such as used in a WLAN that conforms to the IEEE 802.11 standard.
Aspects of the invention are applicable to an RF receiver having an architecture that includes gain control at a plurality of locations in the receive signal path, one or more filters in the signal path, and an analog-to-digital converter to produce digital signals. Thus, aspects of the invention are applicable to a superheterodyne radio receiver, a direct conversion receiver, and to other wireless receiver architectures. The invention, however, will first be explained for a superheterodyne radio receiver.
FIG. 1 shows a typical prior art superheterodyne receiver 100 that includes automatic gain control (AGC). The receive signal path of receiver 100 typically includes a first downconverter 111 to convert the signal at radio frequency to an intermediate frequency (IF) using a signal from a first local oscillator 121, an IF filter 107 in the IF section of the path, and a second downconverter 105 using a signal from a second local oscillator 123 to convert the filtered IF signal at the intermediate frequency to either a baseband signal or a signal at a second intermediate frequency that is close to baseband. Both these cases are called “baseband” herein. The receiver 100 also includes an analog to digital converter to convert the baseband signal to digital signals. These digital signals denoted RxIN are input to a digital modem part of the receiver (not shown). The receive signal path also includes one or more variable gain amplifiers (VGAs) that are set by an automatic gain control (AGC) system 125 to set the gains of the VGAs according to the strength of the received signal. In this case, the front-end in the RF section is adjustable e.g., using a variable gain low noise amplifier (LNA) 103. In addition, there is an IF variable gain amplifier (VGA) 109 in the IF section of the path, and a baseband VGA 113 in the baseband section of the receive signal path. The signal strength is typically measured at the end of the receive signal path, e.g., at the baseband section by a received signal strength indication (RSSI) detector 117.
Consider prior art receiver 100 and suppose the RSSI indicates a weak signal. The gains—in this case the three gains denoted GainRF, GainIF, and GainBB—need to be set to provide sufficient gain in the front-end for the weak signal, but not too high to ensure that the front-end is not overloaded, which might happen if the signal was received in the presence of a powerful interferer in a nearby channel. The interferer signal is filtered by the filter 109 so does not appear in the RSSI. Thus, the RSSI measure of prior art receiver 100 provides no knowledge of whether or not an interferer was present.
A typical prior art AGC circuit is set assuming worst case scenario. That is, the front-end gain GainRF is set to accommodate a worse case adjacent channel interferer, even though such an interferer may not be present.
The need to not set the front-end gain too high limits the performance of the receiver. In particular, having a low gain at the front-end increases the noise that accompanies the desired signal.
Thus there is a need for a method to set the gains that provides for determining whether or not an adjacent interferer is present. Thus there also is a need for a receiver that includes an automatic gain control circuit that adapts to the signal condition by using signal strength indications that indicate the strength of the signal of interest compared to the strength of the signal and adjacent interferers.